The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Operating an internal combustion engine under lean of stoichiometry conditions can improve fuel efficiency, but may increase nitrides of oxygen (NOx) emissions. Such engines include both compression-ignition and lean-burn spark-ignition engines. Aftertreatment devices are known, for instance, utilizing catalysts to treat the exhaust gas flow and catalysts capable of storing some amount of NOx (NOx traps, NOx adsorbers). Engine control technologies have been developed to combine these NOx traps or NOx adsorbers with fuel efficient engine control strategies to improve fuel efficiency and still achieve acceptable levels of NOx emissions. One exemplary strategy includes using a NOx trap to store NOx emissions during fuel lean operations and then purging the stored NOx during fuel rich, higher temperature engine operating conditions with conventional three-way catalysis to nitrogen and water. However, catalysts and NOx traps are dependent upon properties of the exhaust gas to operate efficiently. These methods can be temperature and engine range limiting. A selective catalytic reduction device (SCR) is known to additionally treat the exhaust gas flow utilizing a reductant, extending the aftertreatment capabilities of the aftertreatment system.
One known configuration of SCR utilizes ammonia derived from urea injection or recovered from normal operation of a three-way catalyst device as a reductant to treat NOx. Another known configuration utilizes a hydrocarbon selective catalytic reduction device (HC-SCR), wherein unburnt hydrocarbons, either injected in the exhaust gas flow or carried through from the combustion chamber, are utilized as a reductant to treat NOx. In either method, accurate dosing of the reductant is important to proper function of the device.